config-host.mak: rename confdir to qemu_confdir

config-host.mak: rename confdir to qemu_confdir

$confdir is a confusing name, as it's not clear if it's "the system
config dir" or "the Qemus-specific directory inside the config dir".
$qemu_confdir makes it more clear.

The current C code that uses CONFIG_QEMU_CONFDIR should be safe, as now
create_config generates the same #define name (CONFIG_QEMU_CONFDIR) for
both "confdir" and "qemu_confdir" variables.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>